The present invention relates generally to a method and apparatus for determining a time reference in a cellular communication system. More particularly the present invention relates to expeditiously synchronizing a Global Positioning System (GPS) receiver with its associated GPS satellites and as a result decreasing the device location acquisition rate.
Traditionally, wireless communication devices have functioned solely as a communication device. However, in light of emergency situations and the increased need for location based services in general, the need to accurately locate the subscriber unit (SU) within a given geographical area has become a necessity. Calls originating from a SU under an emergency context can cause a problem for the dispatcher, as the individual using the SU often does not know his exact location. This can have a substantial effect on the time it takes emergency personnel to reach the distressed individual and in life and death situations, time may make a substantial difference. Other location based services, although not as critical as in emergency dispatch, can benefit from the capability to accurately locate the SU.
It is desirable to obtain and communicate physical locations of wireless communication devices within a system, such as radiotelephone handsets within a cellular communication system. In addition, the United States Federal Communications Commission (FCC) has required that cellular communication handsets must be geographically locatable by the year 2001. This capability is desirable for emergency systems such as Enhanced 911 (E911). The FCC requires stringent accuracy and availability performance objectives and demands that cellular communication handsets be locatable within 100 meters 67% of the time for network based solutions and within 50 meters 67% of the time for handset based solutions.
Current generations of cellular communication devices have only limited SU location determination capability. In one technique, the position of the SU is determined by monitoring SU transmissions at several base stations. From time of arrival measurements, the SU's position can be calculated. However, the precision of this technique is limited and, at times, may be insufficient to meet FCC requirements.
Another method and apparatus for determining the location of a SU is to incorporate a Global Positioning System (GPS) receiver into the SU. The GPS receiver is capable of receiving signals from a GPS satellite constellation in a high earth orbit. The GPS receiver is coupled to the microprocessor of the SU and provides location data thereto. This location data may then be transmitted over the cellular communication system from the SU to a base station and then further onto the emergency service requesting the information.
Location determination for GPS is based on triangulation calculations measuring the distance the signal travels from the GPS satellite to the GPS receiver. This method requires the GPS receiver in the SU to have the same time reference as the GPS satellite sending the signal. The signal sent by the GPS satellite includes its time reference along with other information including satellite ephemeris information. Having the same time reference allows the SU to determine how long the signal traveled in time from the GPS satellite to the GPS receiver. Since the signal travels at the speed of light, the distance traveled can be calculated knowing the time it took the signal to reach the SU. However, the SU must initially have a absolute device time in order to rapidly synchronize with the GPS system. This is exacerbated with infrastructure aided GPS systems which use a one second integration time. The absolute device time must be within 10 milliseconds for the GPS calculations to be effective in determining the device location. Acquisition based on time reference acquisition can take an extensive amount of time as the receiver must search all possible code phases. This is because there are multiple satellites sending data at the same time, which are constantly moving at a high rate of speed, in addition to atmospheric aberrations that disrupt the signal as it travels from the satellite down to the SU. This acquisition time may be in terms of minutes to establish the location of the device. In an emergency situation, this time to acquire the time reference and subsequently the location information is unacceptable as it leads to large delays in dispatching the appropriate help and in life and death situations this time may be critical.
Finally, the Iridium.TM. satellite communication system has the capability to locate a SU to within 10 kilometers using a passive geo method for determining the SU location without a GPS receiver. This system too, is clearly unacceptable in terms of accuracy required for the location based services.
Accordingly, a system is needed to improve the time to determine the location of a SU. This is the case not only for emergency situations but for location based services which can improve service with the capability to receive expedited and accurate location information.